1. Field of the Invention
This invention relates to a process for treating calcined gypsum (such as calcium sulfate hemihydrate or stucco) for use in gypsum board manufacture and in making bagged industrial plasters.
More particularly the present invention relates to improvements in those processes in which continuously calcined gypsum stucco is rapidly treated with a small amount of water and allowed to heal so as to lower the high water demand for mixing stucco to produce a fluid slurry suitable for casting and subsequent hydration to gypsum dihydrate.
2. Description of the Prior Art
U.S. Pat. Nos. 4,117,070; 4,153,373 and 4,201,595 describe various apparatus and processes for the rapid water treatment and healing of stucco so as to lower the water demand of the material. Each of these processes teaches a "healing time" which must be allowed so that the small amount of free water blended onto the surface of the particles may fuse the crystallite fragments and fissures in the stucco particles to resist subsequent rapid disintegration into micron sized fractions upon the subsequent mixing of the treated material with gauging water.
Recently issued U.S. Pat. No. 4,238,455 relates to a further improvement; disclosing an apparatus for combining the rapid water treatment step with a grinding step to increase the surface area of the treated particles and redevelop stucco activity while blending the water and stucco. This process also calls for a healing time before the treated material may be mixed with water for casting and hydration to gypsum dihydrate.
Further, recently issued U.S. Pat. No. 4,252,568 discloses an improvement wherein the calcined gypsum is treated with a lignin solution rather than plain water; but again a healing time is required in the treatment.
All of the above processes necessitate a holding time during which the treated stucco particle is healed. To insure that full healing has been obtained, about 5 to more than 20 minutes waiting time is required before the treated material acquires the beneficial properties of reduced mixing water demand through a healing of the crystal's surface. This requires that means be used to provide for the healing time; as by additional conveyors, holding bins and the like. In addition a water blender and a grinding mill may be required. The water blender, delay screw conveyors and grinding mill are vented, jacketed and heated to prevent steam condensation on the walls of the equipment when the treatment water contacts relatively hot stucco. Therefore these processes require high initial capital investment and high maintenance costs. U.S. Pat. No. 4,238,445 offers a process that reduces capital investment by combining the treatment step with the grinding step and thereby eliminate a separate water blender. However a delay time is still required and the problems of handling the treated damp stucco remain. Substantial savings could be realized e.g., in capital investment, maintenance, and operating costs in handling damp treated stucco if the healing time could be substantially shortened or eliminated.